Multipath reduction system

ABSTRACT

The effects of multipath and other interference signals in communication receivers are reduced by implementing an adaptive array. The invention addresses a signal environment in which the directions of arrival and the time of arrival of the signal of interest and the unwanted multipath or interference signals are unknown. The feedback equation of the LMS adaptive array is changed so that a reference signal is not needed. The system uses the strongest received signal as the signal of interest and rejects the other received signals.

CROSS REFERENCE

This application is related to a U.S. patent application filed by thesame inventor on Jan. 16, 1986, and assigned Ser. No. 819,416 andentitled Anti-multipath Signal Processor.

BACKGROUND

To get high quality reception, communication systems, which includeradio and television, require a strong signal that is not corrupted bynoise or interference. One form of interference that can severelydegrade reception is multipath. Multipath occurs when the transmittedsignal arrives at the receiver simultaneously from more than onedirection. The multiple paths are generally due to reflections of thetransmitted signal from hills, buildings, etc.; they can also be theresult of atmospheric phenomena. The indirect paths are longer than thedirect path, and consequently, the indirect path signals arrive at thereceiver later in time than the corresponding direct path signal. Thismakes them arrive at the receiver with a different phase than the directpath signal, and, consequently, causes distortion in both the phase andthe amplitude of the received signal. This can result in deep signalstrength fades, overlapping data, clicking, etc. Examples of multipathdistortion are ghosts on TV, degraded fidelity in commercial FM stereo,and loss of data in communication links.

Designing the antenna pattern gain characteristics to reject theindirect paths by placing a null in their direction of arrival is one ofthe better approaches to reducing multipath distortion. This eliminatesthe indirect paths altogether. It is easy to accomplish when conditionsare known and do not change. But in most communication situations,conditions do change. The adaptive array has been used to automaticallychange the antenna pattern as the conditions change.

In applying an adaptive array to the general communications problemwhere the direction of arrival (DOA) and the time of arrival (TOA) ofthe signal of interest are unknown, the least means squared erroralgorithm (LMS) is well suited. For optimal results, the LMS adaptivearray requires a reference signal which is a replica of the signal ofinterest.

Generation of the reference signal can pose a problem. In practice, areplica of the transmitted signal is not available at the receiver. Thereference signal must be derived from the adaptive array output signal.Robert Riegler and Ralph Compton (Proceedings of the IEEE, Vol. 61, No.6, June 1973, p. 748) have discussed the application of the adaptivearray to amplitude modulated communications signals, where the adaptivearray output signal is processed to generate a representation of thecarrier of the transmitted signal for use as the reference signal. Butthis approach addresses interference signals, not the multipath problem.

R. T. Compton and D. M. DiCarlo (IEEE Transactions on Aerospace andElectronic Systems, VOL.AES-14, No. 4, July 1978, p. 599) and Y.Bar-Ness(IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-18, No.1, January 1982, p. 115) analyze another adaptive array which uses thearray output to generate the reference signal. But their system wasdesigned to address a signal environment in which the signal of interestis received along with a wideband interference signal. They do notaddress the multipath problem.

Ralph Compton (Proceedings of the IEEE, Vol. 66, No. 3, March 1978, p.289) discusses an adaptive array for communication signals using aspread spectrum technique. The adaptive array uses knowledge of thespreading code to generate a reference signal. August McGuffin (U.S.Pat. No. 4,217,586) has extended this approach by utilizing themultipath in the reference signal generation. The pseudo random (PN)code based reference signal generator can keep lock even in severemultipath fading. But both these approaches require a known PN code bepresent in the transmitted signal to generate a reference signal.

G. H. Persinger (1977 International Conference on Communications, IEEE,Pt. III, Chicago, Ill., 12-15 June, 1977, Pp. 259-262) has used a lowlevel PN code placed in quadrature (90 degrees out of phase) with atransmitted AM signal. It is used to generate the reference signal atthe receiver. The reference generation is dependent on the injection ofthis special signal with a known code.

Peder Hansen (IEEE Transactions on Antennas and Propagation, Vol. AP-29,No. 6 November 1981, p. 836) has placed a special modulated pilot signalin the transmitted signal to be used to generate the reference signal.This technique was used specifically to discriminate against multipath.But it does not work without the special pilot signal.

Gayle Martin (U.S. Pat. No. 4,255,791) uses noise decorrelation togenerate a reference signal for an adaptive array. This method addressesan environment where there is a large interfering signal, not themultipath environment.

Kenneth F. Rilling in U.S. patent application Ser. No. 819,416, filed onJan. 16, 1986, entitled Anti-multipath Signal Processor, has amplitudelimited the adaptive array output signal to generate the referencesignal. This system rejects unwanted multipath and low level noise. Butthis work is limited to a reference signal implementation.

In a related technology, transversal filters (single input adaptivefilters) which reduce TV ghosts by signal processing (not by using theantenna pattern) use the known portions of the transmitted TV signalstructure to generate the reference signal (Shri Goyal, others, IEEETransactions on Consumer Electronics, Vol. CE-26, February 1980).Transversal filters remove the ghosts after the received signal has beendemodulated. But, they require a large number of loops, and they aregenerally microprocessor or computer based. Consequently, they are quitecomplicated and expensive.

An alternative to deriving the reference signal, is the elimination ofthe reference signal altogether by changing the feedback equations. Workalong this line has been performed by John Treichler in a relatedtechnology with a single input adaptive filter for constant modulus(amplitude) signals (John R. Treichler and Brian G. Agee, IEEETransactions on Acoustics, Speech, and Signal Processing, Vol. ASSP-31,No. 2, 1983, P. 459; M. G. Larimore and J. R. Treichler, InternationalConference of Acoustics, Speech, and Signal Processing 1983, Boston, P.13). The Constant Modulus Algorithm (CMA) can be used to remove unwantedmultipath for constant amplitude signals because it exploits theamplitude fluctuations induced by multipath. The CMA approach haslimitations: (1) It only applies to wideband signals; it can not handlenarrowband signals or an unmodulated carrier. (2) It requires arelatively large number of adaptive loops.

To summarize, with the exception of the patent application by KennethRilling, the prior art is limited. It either does not address themultipath problem, it applies to a very limited range of signalclassifications, its approach to the problem is complex, or it requiresspecial tones or codes in the transmitted signal. And consequently, withthe exception of the work by Rilling, there is no effective andinexpensive method of removing multipath interference at thecommunications receiver.

SUMMARY OF INVENTION

The object of this invention is to reduce distortions such as fading,data overlap, multiple images, and clicking caused by multipath incommunication receivers. An adaptive array is used to reject unwantedsignals with spatial filtering by placing an antenna pattern null in thedirection of arrival of the unwanted signals. A second object of thisinvention is to reduce the negative effects of other types of noise andinterference signals with amplitudes less than the amplitude of thedesired signal by rejecting them also. The invention does this for asignal environment in which the TOA and the DOA of the desired signaland indirect path/interference signals are unknown and for which thetransmitted desired signal contains no known codes, pilot signals, orsignal waveform structures. This is accomplished by changing thefeedback equation for the LMS adaptive array so that a reference signalis no longer required.

In addition, feedback equation approximations lead to new feedbackequations, resulting in new CMA filter implementations.

DESCRIPTION OF FIGURES

FIG. 1 is a block diagram of a two element array for the suppression ofmultipath and interference: prior art.

FIG. 2 is a block diagram of a two element adaptive array using an LMSanalog implementation: prior art.

FIG. 3 is a block diagram of an N element CMA adaptive array with tappeddelay lines having M output signals respectively.

FIG. 4 is a block diagram of the feedback function implemented byRilling in the reference signal model: prior art.

FIG. 5 is a block diagram of a first implementation of the feedbackfunction for p=1 and q=2.

FIG. 6 is a block diagram of a second implementation of the feedbackfunction for p=1 and q=2.

FIG. 7 is a block diagram of a first implementation of the feedbackfunction for p=2 and q=2.

FIG. 8 is a block diagram of a second implementation of the feedbackfunction for p=2 and q=2.

FIG. 9 is a block diagram of a first implementation of the approximatefeedback function for p=1 and q=2.

FIG. 10 is a block diagram of a second implementation of the approximatefeedback function implementation for p=1 and q=2.

FIG. 11 is a block diagram of a computer implementation of theinvention.

FIG. 12 is the flowchart of a software CMA adaptive array and filterimplementation of the invention.

FIG. 13 is a flow chart of the approximate feedback function for asoftware implementation for p=1 and q=2.

FIG. 14 is a flow chart of the feedback function for the softwareimplementation for p=1 and q=2.

FIG. 15 is a block diagram of a phase shifter added to the N element CMAadaptive array and CMA filter in FIG. 3.

DETAILED DESCRIPTION

Before describing the preferred embodiment of the invention in detail, adiscussion of multipath theory, adaptive arrays, and the new feedbackequation theory of the class of adaptive arrays and filters used in thisinvention to solve the multipath problem will be presented to facilitateunderstanding.

NATURE OF MULTIPATH

In a multipath environment the transmitted signal arrives at thereceiver from several directions simultaneously where there is a directpath and one or more indirect paths. The indirect paths are longer thanthe direct path, so the signals traveling these paths arrive at thereceiver at a later time than the direct path signal. It is thisdifference in the time of arrival that causes distortion in both theamplitude and the phase of the received signal. For example, considerangle modulation (FM, PM, etc.); the direct path signal, in realnotation, is

    X.sub.1 (t)=B.sub.1 sin [w(t-R.sub.1 /c)+αf(t-R.sub.1 /c)]+n.sub.1 (t)                                                       (1)

where w is the angular frequency, t is the time, f(t) is the modulation,B₁ is a constant amplitude, R₁ is the path length, c is the speed oflight, α is the phase deviation, and n₁ (t) is a random noise term. Theindirect path signal has the form

    X.sub.i (t)=B.sub.i sin [w(t-R.sub.i /c)+αf(t-R.sub.i /c)]+n.sub.i (t)                                                       (2)

where the X_(i) (t) indicates the "i"th path signal, B_(i) is a constantsignal amplitude for the "i"th path, R_(i) is the distance traveled bythe "i"th path signal, and n_(i) (t) is a random noise term. The n_(i)(t) and n₁ (t) are all independent. The X_(i) (t)'s are all delayedversions of the direct path signal. The total signal present at a givenpoint in space is the sum of the direct and indirect path signals. Usingequations (1) and (2), the total received signal can be written as##EQU1## In equation (3), for mathematical convenience, the term X₁ (t)has subscript one and refers to the direct path signal, the X_(i) (t) inthe summation, where i=2 to i=N, refers to the indirect paths signals(or the interference signals). Summing over sinusoids, and forconvenience assuming that the noise terms are small and can beneglected, equation (3) can be written as

    E(t)=A(t) sin [wt+a(t)]                                    (4)

where ##EQU2## and

    P.sub.i =-(wR.sub.i /c)+αf(t-R.sub.i /c).

It should be noted that if equation (4) represents the net signalpresent at an antenna array phase center, it can be immediately seenthat the net signal received at each antenna element is differentbecause the distance traveled, R_(i), for the received signals isdifferent for each antenna element.

ADAPTIVE ARRAY

Interference signals and multipath create different signal environmentsfor a communications receiver. Multipath occurs when the transmittedsignal of interest arrives at the receiver simultaneously from more thanone direction. An interference source is a signal source unrelated tothe communications system, such as the signal from another transmitter,that may or may not have the same frequency as the signal of interest.Historically, adaptive arrays were developed to reject externalinterference signals. More recently, adaptive arrays have been showncapable of rejecting multipath.

An adaptive array is an antenna array that has adjustable weights ineach of the antenna elements which automatically adjusts the weights sothat the multipath or interference signals are rejected. The weights canbe amplitude scale factors multiplying the antenna element signals orimplementations that are equivalent to this.

To demonstrate the way in which an array with adjustable weights canreject an indirect multipath signal or an interference signal, considerthe two element array in FIG. 1. Let antenna elements 10 beomni-directional and let the spacing between them be a half-wave lengthof the desired signal.

The desired signal, P(t), arrives from the normal direction, 0 degrees,and the multipath or interference signal I(t) arrives from 30 degreesdisplaced from the desired signal. To simplify the calculation, let bothP(t) and I(t) have zero phase at the array phase center, PC, which islocated midway between the antenna elements. The output signal of eachantenna element 10 goes to a variable complex weight 26", where W₁ +jW₂and W₃ +jW₄ correspond to elements E1 and E2 respectively. The complexweights output signals are summed in adder 30, the output of which isthe array output signal.

The signal of interest, in complex notation, is

    P(t)=P.sub.o exp (jwt),                                    (5)

where P_(o) is the signal amplitude, t is time, and w is the signalangular frequency. The array output signal due to the signal of interestis

    SI(t)=P.sub.o {(W.sub.1 +W.sub.3)+j(W.sub.2 +W.sub.4)} exp (jwt). (6)

The desired array output signal is an unaltered copy of the signal ofinterest. By equating equations (5) and (6), and collecting the real andimaginary terms, the required weight relationships to get the desiredoutput signal are

    W.sub.1 +W.sub.3 =1                                        (7)

and

    W.sub.2 +W.sub.4 =0.                                       (8)

The unwanted indirect path signal is

    I(t)=I.sub.o exp (jwt)                                     (9)

where I_(o) is the signal amplitude. The distance traveled by thereceived signal is different for each antenna element. I(t), which isincidenting the antenna array from an angle of 30 degrees, will arriveat antenna element E2 with a phase lead relative to the antenna arrayphase center of

    θ=2(1/4) sin (30)=π/4                             (10)

radians and, similarly, it will arrive at antenna element E1 with aphase lag of θ=-π/4 radians. Therefore, the array output signal due toI(t) is

    SM(t)=I.sub.o {[W.sub.1 +jW.sub.2 ] exp [j(wt-π/4)]+[W.sub.3 +jW.sub.4 ] exp [j(wt+π/4)]}.                                      (11)

Since it is desired to reject the unwanted multipath signal, equation(11) must equal zero. By using the relationships

    exp (-jπ/4)=(1/√2)(1-j)                          (12)

and

    exp (jπ/4)=(1/√2)(1+j)                           (13)

and collecting the real and imaginary terms, equation (11) gives

    W.sub.1 +W.sub.2 +W.sub.3 -W.sub.4 =0                      (14)

and

    -W.sub.1 +W.sub.2 +W.sub.3 +W.sub.4 =0.                    (15)

The weights must satisfy equations (14) and (15) to reject the multipathsignal.

Equations (9), (10), (14), and (15) give 4 equations and 4 unknowns.Solving for the weights gives

    W.sub.1 =0.5, W.sub.2 =-0.5, W.sub.3 =0.5, W.sub.4 =0.5.   (16)

With these weight values the antenna array will accept the signal ofinterest, P(t), and reject the unwanted multipath signal, I(t). Thearray is functioning as a spatial filter.

In an adaptive array the weights are changed automatically to thecorrect values that reject the unwanted multipath/interference signalsand accept the signal of interest. As the signal environment changes,the weights adapt to continue rejecting the multipath/interference. Tobe an adaptive array, the simple array in FIG. 1 requires a means forautomatically changing the weights.

There are a number of approaches for changing the array weightsautomatically. Many examples of adaptive arrays can be found in: RobertA. Monzingo and Thomas W. Miller, Introduction to Adaptive Arrays, JohnWiley & Sons, New York, 1980; Bernard Widrow and Samuel D. Stearns,Adaptive Signal Processing, Prentice-Hall, 1985; and C. F. N. Cowan andP. M. Grant Eds., Adaptive Filters, Prentice-Hall, Inc., 1985.

The Least Means Square (LMS) adaptive array, which requires a referencesignal, is the best known and the best understood approach toautomatically adjust the weights. It is also the simplest to implement.

In the LMS adaptive array the difference between the array output signaland the reference signal is called the error signal, ε, and is used as ameasure of merit in a least means squares sense to adapt the weights byminimizing ε². The basic theory and technology for the LMS adaptivearray has been presented by Bernard Widrow, Proceedings of the IEEE,Vol. 55, No. 12, December 1967, p. 2143 and by Ralph Compton,Proceedings of the IEEE, Vol. 61, No. 6, June 1973, P. 748. The bookscited in the previous paragraphs also present much theory about LMSadaptive array.

FIG. 2 shows a two element adaptive array using an LMS implementation.After the received signals, which include the signal of interest andmultipath/interference, enter the antenna elements 10, each elementsplits the signal into two components; one component is phase shifted 90degrees by 20', and the other component's phase is unshifted. Eachsignal then goes to its respective amplitude weight 26, which are W₁,W₂, W₃, and W₄ respectively. Because the signals going to each of therespective antenna element weight pairs are 90 degrees out of phase theyadjust the signal in the element in both amplitude and phase. Forelement E1, the amplitude weighting is ##EQU3## and the phase shiftweighting is

    φ.sub.w =-tan.sup.-1 (W.sub.1 /W.sub.2).               (17b)

Element E2 has a similar result for weights W₃ and W₄. The weightedsignals from weights W₁, W₂, W₃, and W₄ go to adder 30 where they aresummed. The output signal of the adder 30 is the adaptive array outputsignal and it goes to subtractor 34. The second input signal tosubtractor 34 is the reference signal, which, ideally, is a replica ofthe desired signal. The array output signal is subtracted from thereference signal by subtractor 34. It is this resulting difference εbetween the array output signal and the reference signal that is used inthe LMS adaptive arrays to automatically adjust the weights.

It can be shown that

    dW.sub.i /dt=-k∇W.sub.i (<ε.sup.2 >)i=1, . . . , N (18a)

where

W_(i) is the "i"th weight, k is a constant,

W_(i) (<ε² >) is the component of the gradient of <ε² > with respect toW_(i) and < > denotes the time average of the function containedtherein. This gives for the value of the "i"th weight ##EQU4## whereW0_(i) is the value of the "i"th weight at time zero, and X_(i) is theinput signal to the "i"th weight. Equations (18b) are the feedbackequations for the weights in the analog implementation. The error signalε from subtractor 34 and the weight input signals X₁, X₂, X₃, X₄ aremultiplied by multipliers 22 respectively. The output signals frommultipliers 22 go to integrators 24 respectively. The output signals ofeach of the integrators 24 is applied to its associated weight circuit26, where that signal is weighted. The output signal from each weightcircuit is then applied to adder 30 where they are summed. Each set ofmultiplier, integrator, weight circuit and input signal together withthe error signal, subtractor, and adder constitute an adaptive loop.

The equivalent feedback equation for a discrete/digital implementationof the LMS adaptive array is

    W.sub.i (j+1)=W.sub.i (j)-2k∇Wi(<ε(j).sup.2 >)i=1, . . . N (19a)

and

    W.sub.i (j+1)=W.sub.i (j)-2kε(j)X.sub.i (j)i=1, . . . , N (19b)

where the antenna element input signals are discrete time samples withX_(i) (j) being the "i"th antenna element input signal at the "j"th timesample, ε(j) is the error signal at the "j"th time sample, W_(i) (j) isthe amplitude weight for the "i"th antenna element input signal at the"j"th sample, and W_(i) (j+1) is the weight value update at the "j+1"time sample for the "i"th antenna element input signal.

The adaptive array is not restricted to two antenna elements and a 90degree phase delay. It can have many antenna elements. And it can havemany time (phase) delays in each antenna element.

CMA ADAPTIVE ARRAYS

The LMS adaptive array minimizes the mean square error between the arrayoutput signal and a reference signal. The CMA filter developed byTreichler minimizes a positive definite measure of the signal modulusvariation given by

    J.sub.pq (t)=<||Y(t)|.sup.p -δ.sup.p |.sup.q >                                        (20)

where "p" and "q" are constants, δ is a positive constant, and Y(t) isthe adaptive filter output signal at time t. The feedback equation forthe "i"th weight is

    W.sub.i (t)=W0.sub.i -2k∫∇.sub.Wi {J.sub.pq (t)}dt (21)

where k and W0_(i) are constants and ∇_(Wi) {J_(pq) (t)} is thecomponent of the gradient of J_(pq) (t) with respect to W_(i). It can beshown that

    ∇.sub.Wi J.sub.pq =<qpX.sub.i (t)Y(t)|Y(t)|.sup.p-2 (|Y(t)|.sup.p -δ.sup.p).sup.q-1 [sgn (|Y(t)|.sup.p -δ.sup.p)].sup.q >                                  (22)

where X_(i) (t) is the input signal to the "i"th weight and ##EQU5##

The feedback equations can be rewritten in the form

    W.sub.i (t)=W0.sub.i -2k∫<X.sub.i (t)ε>dt     (23)

where ε is determined from equations (21) and (22). Table I shows ε fordifferent values of p and q.

                  TABLE I                                                         ______________________________________                                        p,q                            Eq #                                           ______________________________________                                        1,1  {Y(t)/|Y(t)|}sgn[|Y(t)| -                 δ]                  (24)                                           1,2  2{Y(t)/|Y(t)|}[|Y(t)| - δ]                                    (25)                                           2,1  2Y(t)sng{|Y(t)|.sup.2 - δ.sup.2 }                                               (26)                                           2,2  4Y(t){|Y(t)|.sup.2 - δ.sup.2 }                                                  (27)                                           1,3  3{Y(t)/|Y(t)|}{|Y(t)| - δ}.         sup.2 sgn[|Y(t)| - δ]                                                           (28)                                           1,4  4{Y(t)/|Y(t)|}{|Y(t)| - δ}.         sup.3                     (29)                                           3,1  3Y(t)|Y(t)| sgn{|Y(t)|.sup.3 -            δ.sup.3 }           (30)                                           3,2  6Y(t)|Y(t)|{|Y(t)|.sup.3 -                δ.sup.3 }           (31)                                           ______________________________________                                    

Equations of ε for values of "p" and "q" other than those shown in TableI have similar but more complicated form.

Feedback equation (25) is mathematically the same, within a sign andscale factor, as the equation obtained for the error signal in an LMSadaptive array that generates its reference signal by amplitude limitingthe adaptive array output signal (Kenneth Rilling, U.S. patentapplication Ser. No. 819,416).

The adaptive array implementation of equation (25) results in a meansfor removing multipath that is very different from the CMA filterimplementation:

(1) The CMA filter exploits the fact that for a constant modulus signal,multipath causes the amplitude to fluctuate significantly when thesignal has a wide bandwidth. The LMS adaptive array is a spatial filterthat also exploits the different directions of arrival the multipathsignals.

(2) The CMA filter uses scaled, time shifted versions of the receivedinput signal to remove unwanted multipath. The LMS adaptive arrayapproach removes the unwanted multipath signals by placing an antennapattern null in their direction of arrival.

(3) The CMA filter applies to wideband signals only. The LMS adaptivearray approach applies to unmodulated carriers, narrowband signals, andwideband signals.

(4) The CMA filter requires a large number of adaptive loops. The LMSadaptive array approach can use as few as four linear adaptive loops(two antenna elements, each having two linear adaptive loops).

(5) The CMA filter applies to a single signal input adaptive filter. TheLMS adaptive array approach applies to multiple signal inputs from anantenna array.

(6) The CMA filter does not use a reference signal. The LMS adaptivearray uses a reference signal that is generated by amplitude limitingthe adaptive array output signal.

(7) The CMA filter was derived by a whole new theory. The LMS adaptivearray uses the traditional LMS theory.

(8) The CMA filter applies primarily to signals of constant modulus. TheLMS adaptive array approach does not have this limitation.

Since the CMA feedback equation (25) occurred in the LMS adaptive arraywhich removes unwanted multipath, it implies that the other CMA filterfeedback equations obtained from equations 21 and 22 can also be used inan adaptive array to remove unwanted multipath. And it is theapplication of these feedback equations to an adaptive array that makesthe invention different from the prior art. Just like the LMS adaptivearray implementation of equation (25), the adaptive array implementationof the CMA feedback equations places an antenna pattern null in thedirection of arrival of unwanted multipath/interference signals; thesenew CMA adaptive arrays work for broadband signals, narrowband signalsand unmodulated carriers; they also work for signals without constantmudulus such as AM signals. In addition, they require as few as fourlinear adaptive loops (two antenna elements and two linear adaptiveloops for each antenna element).

By comparing equation (23) to the LMS feedback equation, equation (18b),it is seen that the CMA and LMS feedback equations have the same form.Only the definition of is different. ε for the LMS adaptive array isgiven by the difference between the adaptive array output signal and thereference signal. ε for the CMA feedback equations is derived fromequations (21), (22), and (23). This means that the form of the weightadjustment is the same for the CMA adaptive array and LMS adaptive arrayexcept for the computing of ε.

FIG. 3 shows the generalized adaptive array implementation based on theCMA feedback equation (23) and the LMS feedback equation (18b). Theinput signals to the adaptive array are developed by appropriate inputdevices such as antenna elements 10, bandpass filters 12, and mixers 18.The second input signal to the mixers 18 is generated by a single localoscillator 16. Thus, mixers 18 convert the input signals to phasecoherent signals at an appropriate IF frequency. The respective mixer 18output signals go to a corresponding tapped delay line 20 which has Moutput terminals. Each tapped delay line 20 output signal goes to acorresponding amplitude weight circuit 26 and multiplier 22.

The output signal from each amplitude weight circuit 26 is applied toadder 30 where they are summed. The adder 30 output signal is theadaptive array output signal which goes to the appropriate next stage ofsignal processing, such as an IF amplifier, a demodulator, etc.

The output signal of adder 30 is also applied to feedback functioncircuit (FF) 32 which computs the feedback signal ε in equation (23).The form of FF 32 depends on the specific equation for ε being used andits particular implementation. Table I shows the equations of ε for somevalues of p and q. Specific implementations of FF 32 are presentedbelow.

The output signal from FF 32 is applied to each multiplier 22. Eachmultiplier 22 multiplies the feedback signal from FF 32 with thecorresponding output signal from tapped delay line 20. The output signalfrom each of multipliers 22 is applied to a corresponding integrator 24.The output signal of integrator 24 is applied to a correspondingamplitude weight circuit 26, which, accordingly, adjusts the weightvalues applied to the output signal of its corresponding tapped delayline 20. This weight adjustment process continues until the weightsreach equilibrium values. The system adjusts the weights so that theunwanted multipath/interference signals are rejected, resulting in lessdistortion of the signal of interest at the adaptive array outputsignal.

FIG. 15 shows a phase shifter 86 placed between FF 32 and multipliers 22of FIG. 3. The output signal of FF 32 is applied to phase shifter 86.The output signal of phase shifter 86 is then applied to multipliers 22.The system functions in the same manner as the system in FIG. 3 exceptthat the phase shifter 86 can shift the phase of the feedback signal tooptimize the stability of the system if necessary, because as the phaseis removed from its optimum value, the system can exhibit a drift.

It will be seen from the material presented below that although thereare many possible values of p and q and that each pair of p and q valuescan have many implementations, there are only a handful of fundamentalimplementations. The other implementations are more elaborate versionsof these fundamental implementations.

As discussed above, equation (25) can be implemented as an LMS adaptivearray with the reference signal being generated by amplitude limitingthe adaptive array output signal. This is an implementation of equation(25) where the equation is separated into to the two terms: Y(t), theadaptive array output signal, and {Y(t)/|Y(t)|}, the generated referencesignal. The FF 32 for this implementation is shown in FIG. 4. Theadaptive array output signal from adder 30 in FIG. 3 goes to amplitudelimiter 36 and subtractor 34. Amplitude limiter 36 amplitude limits theadder 30 output signal. The output signal of amplitude limiter 36 is thesecond input signal to subtractor 34. Subtractor 34 subtracts the outputsignal of adder 30 from the output signal of amplitude limiter 36. Theoutput signal of subtractor 34 is the feedback signal ε and is an inputsignal to multipliers 22 in FIG. 3 (or phase shifter 86 in FIG. 15). Inpractice, to make equation (25) equivalent, to the prior art, therequired sign change can be implemented in many other ways besides usingthe subtractor as given above. This implementation is prior art.

Equation (25) can also be separated into the two factors 2{Y(t)/|Y(t)|}and {|Y(t)|-δ}. FIG. 5 and FIG. 6 show two FF 32 implementations of thisform. In FIG. 5 the output signal of adder 30 is applied to a biasedenvelope detector 40 and an amplitude limiter 38. The amplitude limiter38 amplitude limits the adder 30 output signal. The amplitude limiter 38output signal goes to multiplier 42. The biased envelope detector 40detects the envelope of the output signal of adder 30 and biases it aconstant negative value. The output signal of biased envelope detector40 is the second input signal to multiplier 42. The multiplier 42multiplies the output signal of amplitude limiter 38 and the outputsignal of biased envelope detector 40. The output signal of multiplier42 is the feedback signal ε and is applied to the multipliers 22 in FIG.3 (or phase shifter 86 in FIG. 15). This form of the adaptive array isone embodiment of the present invention.

The second implementation is given in FIG. 6. The output signal fromadder 30 in FIG. 3 is applied to the envelope detector 44 and thedivider 46. The envelope detector 44 detects the envelope of the outputsignal of adder 30. The output signal of envelope detector 44 is appliedto the biaser 48 and the divider 46. Biaser 48 shifts the output signalof envalope detector 44 a constant negative amount. The output signal ofbiaser 48 is one of the input signals to multiplier 42. Divider 46divides the output signal from adder 30 by the output signal fromenvelope detector 44. The output signal of divider 46 is applied tomultiplier 42. Multiplier 42 multiplies the output signals from divider46 and biaser 48. The output signal of multiplier 42 is the feedbacksignal ε and is applied to multiplier 22 in FIG. 3 (or phase shifter 86in FIG. 15). This form of the adaptive array is another embodiment ofthe present invention.

Equation (24) can be implemented by extending the implementations ofequation (25). A sgn means, such as a comparator referenced to zerovolts, can be added to the implementation in FIG. 5 by having the theoutput signal of biased envelope detector 40 applied to the sgn means;and the output signal of sgn means is applied to multiplier 42.Similarly, equation (24) can be implemented through the addition of asgn means to the implementation of equation (25) in FIG. 6 by having theoutput signal of biaser 48 applied to the sgn means, and the outputsignal of sgn means is applied to multiplier 42.

Equation (27) can be separated into the two factors Y(t) and {|Y(t)|²-δ² }. FIG. 7 shows an implementation of this form. The output signal ofadder 30 in FIG. 3 is applied to multiplier 42 and to envelope detector44. Envelope detector 44 detects the amplitude envelope of the outputsignal of adder 30. The output signal of envelope detector 44 is appliedto both input terminals of multiplier 50. Multiplier 50, so connected,squares the output signal of envelope detector 44. The output signal ofmultiplier 50 is applied to biaser 48. Biaser 48 shifts the outputsignal of multiplier 50 a constant negative amount. The output signal ofbiaser 48 is the second input signal of multiplier 42. Multiplier 42multiplies the output signals of biaser 48 and adder 30. The outputsignals of multiplier 42 is the feedback signal ε and is applied tomultipliers 22 in FIG. 3 (or phase shifter 86 in FIG. 15). This form ofthe adaptive array is another embodiment of the present invention.

Equation (27) can also be separated into the two terms 4|Y(t)|² Y(t) and-4δ² Y(t). FIG. 8 shows an implementation where ε has been chosen to beequal to 1. The adaptive array output signal from adder 30 is applied toenvelope detector 44 and subtractor 54. Envelope detector 44 detects theenvelope of the output signal of adder 30. The output signal of envelopedetector 44 is then applied to both input terminals of multiplier 50,which, as connected, squares the output signal of envelope detector 44.The output signal of multiplier 50 is also applied to multiplier 42.Multiplier 42 multiplies the output signal of multiplier 50 and theoutput signal of adder 30. The output signals of multiplier 42 isapplied to subtractor 54. Subtractor 54 subtracts output signal of adder30 from output signal of multiplier 42. The output signal of subtractor54 is the feedback signal ε and it is applied to the multipliers 22 inFIG. 3 (or phase shifter 86 in FIG. 15). This form of the adaptive arrayis yet another embodiment of the present invention.

Equation (26) can be implemented by extending the implementation in FIG.7 by placing a sgn means, such as a comparator referenced to zero volts,between the biaser 48 and the multiplier 42. The output signal of biaser48 is applied to the sgn means, and the output signal of sgn means is inturn applied to multiplier 42.

The feedback equations corresponding to the other values of p and q areimplemented by adding more multipliers, biasers, etc., to the FF 32circuits already presented. They are extensions of the forms presentedabove.

The significant differences between these CMA adaptive array inventionsand the CMA filters are:

(1) The CMA filter exploits the fact that for a constant modulus signalmultipath causes the amplitude to fluctuate significantly when thesignal has a wide bandwidth. The CMA adaptive array is a spatial filterthat also exploits the different directions of arrival of the multipathsignals.

(2) The CMA filter uses a scaled, time shifted version of the receivedinput signal to remove unwanted multipath. The CMA adaptive arrayapproach removes the unwanted multipath signals by placing an antennapattern null in their direction of arrival.

(3) The CMA filter applies only to wideband signals. The CMA adaptivearray approach applies to unmodulated carriers, narrowband signals, andwideband signals.

(4) The CMA filter requires a large number of adaptive loops. The CMAadaptive array approach can use as few as four linear adaptive loops(two antenna elements, each having two linear weights).

(5) The CMA filter applies to single signal input. The CMA adaptivearray approach applies to multiple signal inputs for an antenna array.

(6) The CMA filter applies primarily to signals of constant modulus. TheCMA adaptive array approach does not have this limitation

APPROXIMATE FEEDBACK EQUATION

Using equations (23) and (25), the feedback equation corresponding toequation (25) is

    W.sub.i =WO.sub.i -k∫<X.sub.i {2[Y(t)/|Y(t)|][|Y(t)|-δ]}>dt (30)

Let equation (32) be approximated by

    W.sub.i =WO.sub.i -{k/|Y(t)|}∫<X.sub.i {2Y(t)[|Y(t)|-δ]}>dt              (3)

where the factor 1/|Y(t)| is moved outside the integral. Feedbackequation (33) can be used to derive a new form for the adaptive array.The feedback signal for equation (33) is

    ε=2Y(t){|Y(t)|-δ}.         (34)

Equation (32) can be separated into two factors: 2Y(t) and [Y(t)-δ].FIG. 9 shows an implementation of FF 34 for this form. The output signalfrom adder 30 is applied to both the baised envelope detector 40 andmultiplier 42. The biased envelope detector 40 detects the amplitudeenvelope of the output signal of adder 30 and shifts it a constantnegative amount. The output signal of biased envelope detector 40 isapplied to the multiplier 42. Multiplier 42 multiplies the output signalof biased envelope detector 40 and output signal of adder 30. The outputsignal of multiplier 42 is the feedback signal and is applied tomultiplier 22 in FIG. 3 (or phase shifter 86 in FIG. 15). This form ofthe adaptive array is still another embodiment of the present invention.

Equation (34) can also be separated into two terms: 2|Y(t)|Y(t) and-2δY(t). FIG. 10 shows an implementation of this two term separation forδ=1. The output signal of adder 30 in FIG. 3 is applied to envelopedetector 44, multiplier 42, and subtractor 34. Envelope detector 44detects the amplitude envelope of the output signal of adder 30. Theoutput signal of envelope detector 44 is applied to multiplier 42.Multiplier 42 multiplies the output signal of envelope detector 44 andthe output signal of adder 30. The output signal of multiplier 42 isapplied to subtractor 34. Subtractor 34 subtracts the output signal ofadder 30 from the output signal of multiplier 42. The output signalsubtractor 34 is the feedback signal ε and is applied to multipliers 22in FIG. 3 (or phase shifter 86 in FIG. 15). This form of the adaptivearray is a further embodiment of the present invention.

When the same approximation is applied to equation (24), the feedbackequation becomes

    ε=2Y(t) sgn {|Y(t)|-δ}.    (35)

Equation (35) can be implemented with an extension of the implementationin FIG. 9 through the addition of a sgn means, such as a comparatorreferenced to zero volts, between the biased envelope detector 40 andmultiplier 42. The output signal of biased envelope detector 40 isapplied to the sgn means; the output signal of the sgn means is appliedto multiplier 42.

The other feedback equations for the different values of p and q with|Y(t)| as a factor can also be approximated by moving the |Y(t)| factorout of the integral in a similar manner.

These new approximations to the feedback equations for the CMA adaptivearray can be applied to the CMA filter as well. FIG. 3 (or phase shifter86 in FIG. 15) would have only a single input signal in this case(besides an antenna input signal, any other input signal made up ofmultiple images of the desired signal is applicable). The implementationof the new approximations of the feedback equations in FIGS. 9 and 10 tothe CMA filters are new inventions.

FIG. 3 (or phase shifter 86 in FIG. 15) with the FF 32 given in FIGS. 4to 10 are some implementations of the new inventions, however, theimplementations of the inventions are not limited to the implementationspresented.

All the above CMA adaptive arrays can be implemented in software,digital, analog and hybrid form.

OTHER NOISE SOURCES

The present CMA adaptive array invention and filter inventionembodiments presented above can also reduce distortion effects caused byexternal interference. When the amplitude of the signal of interest isgreater than the amplitude of the interference signal, the interferencesignal is rejected. This is to be expected because the LMS adaptivearray (Rilling U.S. patent application Ser. No. 819,416) and CMA filterreject the same type of external interference.

HARDWARE

Presented below are manufacturer part/model numbers for the keycomponents of a specific hardware implementation of FIG. 3 using the FF32 implemented in FIGS. 4 to 10. These implementations operate at theintermediate frequency of 10 Mhz after down converting from the receivedfrequency with mixers 18 and local oscillator 16. The tapped delay lines20 can be implemented with a Data Delay Devices 1505-100A tapped delayline. It has equal taps which, when including an undelayed version ofthe antenna element input signal, gives the tapped delay line 20 sixoutput terminals. For narrow bandwidth signals, an alternative to thetapped delay line is the ninety degree hybrid which can be implementedby the Mini-Circuits PSCQ-2-10.5. The multiplier 22 can be implementedwith Mini-Circuits SBL-1 mixer operated in the linear multiplicationregion. The integrator 24 can be implemented with National SemiconductorLH0032 operational amplifier in an integrator circuit configuration. Theamplitude weight circuit 26, multiplier 56, multiplier 50, andmultiplier 42 can be implemented with the Motorola MC1595 four quadrantlinear multiplier. The adder 30 can be implemented from a network ofMini-Circuits MSC-2-1 two way power combiners where the number ofsignals to be summed determines the number of power combiners required.The amplitude limiters 34 and 38 can be implemented by an Avantek, Inc.UTL-1002 signal limiter. The subtractor 54 can be implemented by aMini-Circuits PSCJ-2-1 180 degree two way power combiner. The biaser 48can be implemented by the National Semiconductor LH0032 operationalamplifier and a DC voltage source. The divider 46 can be implemented bythe Motorola MC1595 four quadrant linear multiplier and a NationalSemiconductor LH0032 operational amplifier. The envelope detector can beimplemented by an RF diode as a detector, an RC low pass filter and thebias envelope detector can be implemented by an RF diode as a detector,an RC low pass filter and a National Semiconductor LH0032 operationalamplifier with a DC voltage biasing the output signal of the envelopedetector. For narrowband signals the phase shifter 86 can be implementedby a Data Delay Device 1503-100A variable delay.

These are just one specific set of hardware for implementing the variousembodiments of the present CMA adaptive array and filter inventions,however, the inventions are not limited to the use of these componentsor these specific implementations.

SOFTWARE

FIG. 11 shows the adaptive array which uses a computer, microprocessor,or digital signal processors (DSP). The signal of interest and theunwanted multipath/interference signals are received by the antennaelements 10. Each of the composite signals received by each antenna isapplied to a bandpass filter 12. The output signal of bandpass filter 12is applied to a mixer 14. Each of mixers 14 also receives a second inputsignal which is the output signal of the local oscillator 16. Each ofthe output signals of mixer 14 are applied to an analog to digital (A/D)converter 60. The output signal from each A/D converter 60 is applied tothe computer/microprocessor/DSP 62. The CMA adaptive array algorithmimplemented in the computer/microprocessor/DSP 62. For the CMS filter,only a single input signal will be received by thecomputer/microprocessor/DSP 62.

FIG. 12 shows a flow chart for software implementations of the CMAadaptive arrays and filters, however, the software implementations ofthe various embodiments of the present invention are not limited to theimplementations presented below. Each of the digitized antenna elementsignals from A/D converters 60 go to splitter/delayer 64. Thesplitter/delayer 64 makes copies of each input signal and delays eachcopy an appropriate length of time in such a way that it is the softwareequivalent of an M output tapped delay line; the number of copies ofeach input signal and the magnitude of each time delay depends on thesignal frequencies, signal bandwidth, signal environment, performancerequired, etc. Each signal copy is associated with an adaptive loop. Thesplitter/delayer 64 output signal goes to the weighter 66 where each ofthe input antenna element signals, delayed and undelayed, are weightedby an initial weight value. The splitter/delayer 64 output signal alsogoes to multiplier 72. The weighted signals from weighter 66 are summedin adder 68. The output signal of adder 68 is the adaptive array outputsignal and goes to feedback function 70. Feedback function 70 computesthe feedback signal ε. Which specific feedback equation is implementedby feedback function 70 depends on which values of p and q are chosen inequations (21), (22), and (23) and, where appropriate, whether anapproximation is chosen. Two examples of feedback function 70implementations are presented below. The output signal ε from feedbackfunction 70 goes to multiplier 72. Multiplier 72 multiplies the feedbacksignal ε with each of the delayed and undelayed signals fromsplitter/delayer 64. The multiplied output signals for each adaptiveloop from multiplier 72 goes to integrator 74. Integrator 74 integratesthe output signal of multiplier 72 for each respective adaptive loop.The integrator 74 output signal goes to weighter 66 which updates thevalue of each corresponding weight. This cycle continues to repeatitself with the weight values converging to their equilibrium values.

FIG. 13 shows the flow chart for the feedback function 70 for onesoftware implementation of equation (32) when it is separated into theterms 2Y(t) and 2[Y(t)-δ]. The output signal of adder 68 in FIG. 12 goesto the biased envelope detector 76 and delay 78. The biased envelopedetector 76 determines the signal envelope of the adder 68 outputsignal. One such envelope detector implementation would be a relativepeak detector. The detected envelope signal is biased by a constantnegative value. The biased envelope detector 76 output signal goes tomultiplier 80. When necessary, delay 78 delays the output signal ofadder 68 to account for a time delay required to implement the biasedenvelope detector 76 so that the output signal of biased envelopedetector 76 and the output signal of adder 68 are properly synchronized.The output signal of delay 78 goes to multiplier 80. Multiplier 80multiplies the output signal of biased envelope detector 76 and theoutput signal of delay 78. The output signal of multiplier 80 is thefeedback signal ε and goes to multiplier 72 in FIG. 12.

FIG. 14 shows the flow chart for the feedback function 70 for onesoftware implementation of equation (25) when separated into the twofactors 2{Y(t)/|Y(t)|} and {|Y(t)|-δ}. The output signal of adder 68 inFIG. 12 goes to the biased envelope detector 76 and amplitude limiter82. One implementation of the amplitude limiter is a clamper and lowpassdigital filter.

At each data sample, the clamper assigns to the output signal of adder68 an amplitude of +F if the output signal of adder 30 is positive and-F if the output signal of adder 30 is negative. The constant F is setat a convenient value determined by the amplitude of the input signals,parameter values of other software functional blocks, round off errors,required performance, etc. The clamper output signal is of a rectangularwave form and goes to the digital filter which removes the second andhigher harmonics to convert its rectangular wave form to a sinusoid formof constant amplitude. The resulting output signal from the digitalfilter is an amplitude limited version of the output signal of adder 68.The amplitude limiter 82 output signal goes to delay 84.

The biased envelope detector 76 determines the signal amplitude envelopeof the output signal of adder 68. One such envelope detectorimplementation would be a relative peak detector. The detected envelopesignal is biased by a constant negative value. The output signal ofbiased envelope detector 76 also goes to delay 84.

Delay 84 appropriately delays either the output signal of biasedenvelope detector 76 or the output signal of amplitude limiter 82 sothat the two signals are properly synchronized. Which of the two signalsis actually delayed depends on the details of the specificimplementations of the biased envelope detector 76 and the amplitudelimiter 82. The synchronized output signals of the biased amplitudedetector 76 and of amplitude limiter 82 go from delay 84 to multiplier80. Multiplier 80 multiplies these output signals. The multiplier 80output signal is the feedback signal ε and goes to multiplier 72 in FIG.12.

It would be clear to a person skilled in the art that the adaptive arrayand filter software implemented by the CMA adaptive array and filtersoftware flowchart in FIG. 12, FIG. 13, and FIG. 14 can be implementedby other means.

It would be clear for someone skilled in the art that the invention canbe implemented in either digital, analog/digital hybrid,software/digital hybrid or analog/software hybrid forms.

From the forgoing description, it will be apparent that the inventiondisclosed herein provides novel and advantageous signal processingsystems. It will be understood by those familiar with the art, theinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof.

What is claimed is:
 1. A signal processing system for reducingdistortion effects in communication receivers due to multipath, saidsystem comprising:adaptive array means having:at least two antennaelements; weighting means coupled to the antenna elements forselectively weighting the received signals by a selected weight factor;and summing means for adding together the signals from the weightingmeans for generating an adaptive array output signal; biased envelopedetector means coupled to receive said adaptive array output signal forgenerating the biased amplitude envelope of said adaptive array outputsignal; and first multiplier means coupled to receive biased envelopedetector means output signal and coupled to receive adaptive arrayoutput signal for generating a feedback signal; said adaptive arraymeans also including a second multiplier means for each weighting meanscoupled to receive the feedback signal and coupled to receive thecorresponding weighting means input signal, the output signal of whichis coupled to a corresponding integrator means whose output signal iscoupled to the corresponding weighting means for automaticallyredefining the weight factors.
 2. A system as in claim 1 furtherincludes a phase adjustment means for adjusting the phase of thefeedback signal.
 3. A signal processing system for reducing distortioneffects in communication receivers due to multipath, said systemcomprising:adaptive array means having:at least two antenna elements;weighting means coupled to the antenna elements for selectivelyweighting the received signals by a selected weight factor; and summingmeans for adding together the signals from the weighting means forgenerating an adaptive array output signal; biased envelope detectormeans coupled to receive said adaptive array output signal forgenerating the biased amplitude envelope of said adaptive array outputsignal; amplitude limiter means coupled to receive the said adaptivearray output signal for generating an amplitude limited adaptive arrayoutput signal; and first multiplier means coupled to receive biasedenvelope detector means output signal and coupled to receive amplitudelimiter means output signal for generating a feedback signal; saidadaptive array means also including a second multiplier means for eachweighting means coupled to receive the feedback signal and coupled toreceive the corresponding weighting means input signal, the outputsignal of which is coupled to a corresponding integrator means whoseoutput signal is coupled to the corresponding weighting means forautomatically redefine the weight factors.
 4. A system as in claim 3further means includes a phase adjustment means for adjusting the phaseof the feedback signal.
 5. A signal processing system for reducingdistortion effects in communication receivers due to multipath, saidsystem comprising:adaptive array means having:at least two antennaelements; weighting means coupled to the antenna elements forselectively weighting the received signals by a selected weight factor;and summing means for adding together the signals from the weightingmeans for generating an adaptive array output signal; envelope detectormeans coupled to receive said adaptive array output signal forgenerating the amplitude envelope of said adaptive array output signal;biasing means coupled to receive the envelope detector means outputsignal for biasing the envelope detector means output signal; dividermeans coupled to receive the adaptive array output signal and coupled toreceive the envelope detector means output signal for dividing theadaptive array output signal by the envelope detector means outputsignal; and first multiplier means coupled to receive biasing meansoutput signal and coupled to receive divider means output signal forgenerating a feedback signal; said adaptive array means also including asecond multiplier means for each weighting means coupled to receive thefeedback signal and coupled to receive the corresponding weighting meansinput signal, the output signal of which is coupled to a correspondingintegrator means whose output signal is coupled to the correspondingweighting means for automatically redefining the weight factors.
 6. Asystem as in claim 5 further includes a phase adjustment means foradjusting the phase of the feedback signal.
 7. A signal processingsystem for reducing distortion effects in communication receivers due tomultipath, said system comprising:adaptive array means having:at leasttwo antenna elements; weighting means coupled to the antenna elementsfor selectively weighting the received signals by a selected weightfactor; and summing means for adding together the signals from theweighting means for generating an adaptive array output signal; envelopedetector means coupled to receive said adaptive array output signal forgenerating the amplitude envelope of said adaptive array output signal;third multiplier means coupled to receive biased envelope detector meansoutput signal into both inputs for squaring the envelope detector meansoutput signal; first multiplier means coupled to receive the thirdmultiplier means output signal and coupled to receive the adaptive arrayoutput signal for generating the product of the third multiplier meansoutput signal and said adaptive array output signal; and subtractormeans that is coupled to receive the first multiplier means outputsignal and coupled to receive the said adaptive array output signal forsubtracting the said adaptive array output signal from the firstmultiplier means output signal for generating a feedback signal; saidadaptive array means also including a second multiplier means for eachweighting means coupled to receive the feedback signal and coupled toreceive the corresponding weighting means input signal, the outputsignal of which is coupled to a corresponding integrator means whoseoutput signal is coupled to the corresponding weighting means forautomatically redefining the weight factors.
 8. A system as in claim 7further includes a phase adjustment means for adjusting the phase of thefeedback signal.
 9. A signal processing system for reducing distortioneffects in communication receivers due to multipath, said systemcomprising:adaptive array means having:at least two antenna elements;weighting means coupled to the antenna elements for selectivelyweighting the received signals by a selected weight factor; and summingmeans for adding together the signals from the weighting means forgenerating an adaptive array output signal; envelope detector meanscoupled to receive said adaptive array output signal for generating theamplitude envelope of said adaptive array output signal; firstmultiplier means coupled to receive the envelope detector means outputsignal and coupled to receive the adaptive array output signal forgenerating the product of the envelope detector means output signal andsaid adaptive array output signal; and subtractor means that is coupledto receive the first multiplier means output signal and coupled toreceive the adaptive array output signal for subtracting the adaptivearray output signal from the first multiplier means output signal forgenerating a feedback signal; said adaptive array means also including asecond multiplier means for each weighting means coupled to receive thefeedback signal and coupled to receive the corresponding weighting meansinput signal, the output signal of which is coupled to a correspondingintegrator means whose output signal is coupled to the correspondingweighting means for automatically redefining the weight factors.
 10. Asystem as in claim 9 further includes a phase adjustment means foradjusting the phase of the feedback signal.
 11. A signal processingsystem for reducing distortion effects due to multiple signal images,said system comprising:adaptive filter means having:one signal inputelement; weighting means coupled to the signal input element forselectively weighting the received signal by a selected weight factor;and summing means for adding together the signals from the weightingmeans for generating an adaptive filter output signal; biased envelopedetector means coupled to receive said adaptive filter output signal forgenerating the biased amplitude envelope of said adaptive array outputsignal; and first multiplier means coupled to receive biased envelopedetector means output signal and coupled to receive said adaptive arrayoutput signal for generating the feedback signal; said adaptive filtermeans also including a second multiplier means for each weighting meanscoupled to receive the feedback signal and coupled to receive thecorresponding weighting means input signal, the output signal of whichis coupled to a corresponding integrator means whose output signal iscoupled to the corresponding weighting means for automaticallyredefining the weight factors.
 12. A system as in claim 11 furtherincludes a phase adjustment means for adjusting the phase of thefeedback signal.
 13. A signal processing system for reducing distortioneffects due to multiple signal images, said system comprising:adaptivefilter means having:one input signal element; weighting means coupled tothe input element for selectively weighting the received signal by aselected weight factor; and summing means for adding together thesignals from the weighting means for generating an adaptive filteroutput signal; envelope detector means coupled to receive said adaptivefilter output signal for generating the amplitude envelope of saidadaptive filter output signal; biasing means coupled to receive theenvelope detector means output signal for generating a biased envelopedetector means output signal; divider means coupled to receive theadaptive filter output signal and coupled to receive the envelopedetector means output signal which divides the adaptive filter outputsignal by the envelope detector means output signal; and firstmultiplier means coupled to receive biasing means output signal andcoupled to receive divider means output signal for generating a feedbacksignal; said adaptive filter means also including a second multipliermeans for each weighting means coupled to receive the feedback signaland coupled to receive the corresponding weighting means input signal,the output signal of which is coupled to a corresponding integratormeans whose output signal is coupled to the corresponding weightingmeans for automatically redefining the weight factors.
 14. A system asin claim 13 further includes a phase adjustment means for adjusting thephase of the feedback signal.
 15. A signal processing system forreducing distortion effects in communication receivers due tointerference signals with signal amplitudes less than the signalamplitude of the signal of interest, said system comprising:adaptivearray means having:at least two antenna elements; weighting meanscoupled to the antenna elements for selectively weighting the receivedsignals by a selected weight factor; and summing means for addingtogether the signals from the weighting means for generating an adaptivearray output signal; biased envelope detector means coupled to receivesaid adaptive array output signal for generating the biased amplitudeenvelope of said adaptive array output signal; and first multipliermeans coupled to receive biased envelope detector means output signaland coupled to receive adaptive array output signal for generating afeedback signal; said adaptive array means also including a secondmultiplier means for each weighting means coupled to receive thefeedback signal and coupled to receive the corresponding weighting meansinput signal, the output signal of which is coupled to a correspondingintegrator means whose output signal is coupled to the correspondingweighting means for automatically redefining the weight factors.
 16. Asystem as in claim 15 further includes a phase adjustment means foradjusting the phase of the feedback signal.
 17. A signal processingsystem for reducing distortion effects in communication receivers due tointerference signals with signal amplitudes less than the signalamplitude of the signal of interest, said system comprising:adaptivearray means having:at least two antenna elements; weighting meanscoupled to the antenna elements for selectively weighting the receivedsignals by a selected weight factor; and summing means for addingtogether the signals from the weighting means for generating an adaptivearray output signal; biased envelope detector means coupled to receivesaid adaptive array output signal for generating the biased amplitudeenvelope of said adaptive array output signal; amplitude limiter meanscoupled to receive the said adaptive array output signal for generatingan amplitude limited adaptive array output signal; and first multipliermeans coupled to receive biased envelope detector means output signaland coupled to receive amplitude limiter means output signal forgenerating a feedback signal; said adaptive array means also including asecond multiplier means for each weighting means coupled to receive thefeedback signal and coupled to receive the corresponding weighting meansinput signal, the output signal of which is coupled to a correspondingintegrator means whose output signal is coupled to the correspondingweighting means for automatically redefine the weight factors.
 18. Asystem as in claim 17 further includes a phase adjustment means foradjusting the phase of the feedback signal.
 19. A signal processingsystem for reducing distortion effects in communication receivers due tointerference signals with signal amplitudes less than the signalamplitude of the signal of interest, said system comprising:adaptivearray means having:at least two antenna elements; weighting meanscoupled to the antenna elements for selectively weighting the receivedsignals by a selected weight factor; and summing means for addingtogether the signals from the weighting means for generating an adaptivearray output signal; envelope detector means coupled to receive saidadaptive array output signal for generating the amplitude envelope ofsaid adaptive array output signal; biasing means coupled to receive theenvelope detector means output signal for biasing the envelope detectormeans output signal; divider means coupled to receive the adaptive arrayoutput signal and coupled to receive the envelope detector means outputsignal for dividing the adaptive array output signal by the envelopedetector means output signal; and first multiplier means coupled toreceive biasing means output signal and coupled to receive divider meansoutput signal for generating a feedback signal; said adaptive arraymeans also including a second multiplier means for each weighting meanscoupled to receive the feedback signal and coupled to receive thecorresponding weighting means input signal, the output signal of whichis coupled to a corresponding integrator means whose output signal iscoupled to the corresponding weighting means for automaticallyredefining the weight factors.
 20. A system as in claim 19 furtherincludes a phase adjustment means for adjusting the phase of thefeedback signal.
 21. A signal processing system for reducing distortioneffects in communication receivers due to interference signals withsignal amplitudes less than the signal amplitude of the signal ofinterest, said system comprising:adaptive array means having:at leasttwo antenna elements; weighting means coupled to the antenna elementsfor selectively weighting the received signals by a selected weightfactor; and summing means for adding together the signals from theweighting means for generating an adaptive array output signal; envelopedetector means coupled to receive said adaptive array output signal forgenerating the amplitude envelope of said adaptive array output signal;third multiplier means coupled to receive biased envelope detector meansoutput signal into both inputs for squaring the envelope detector meansoutput signal; first multiplier means coupled to receive the thirdmultiplier means output signal and coupled to receive the adaptive arrayoutput signal for generating the product of the third multiplier meansoutput signal and said adaptive array output signal; and subtractormeans that is coupled to receive the first multiplier means outputsignal and coupled to receive the said adaptive array output signal forsubtracting the said adaptive array output signal from the firstmultiplier means output signal for generating a feedback signal; saidadaptive array means also including a second multiplier means for eachweighting means coupled to receive the feedback signal and coupled toreceive the corresponding weighting means input signal, the outputsignal of which is coupled to a corresponding integrator means whoseoutput signal is coupled to the corresponding weighting means forautomatically redefining the weight factors.
 22. A system as in claim 21further includes a phase adjustment means for adjusting the phase of thefeedback signal.
 23. A signal processing system for reducing distortioneffects in communication receivers due to interference signals withsignal amplitudes less than the signal amplitude of the signal ofinterest, said system comprising:adaptive array means having:at leasttwo antenna elements; weighting means coupled to the antenna elementsfor selectively weighting the received signals by a selected weightfactor; and summing means for adding together the signals from theweighting means for generating an adaptive array output signal; envelopedetector means coupled to receive said adaptive array output signal forgenerating the amplitude envelope of said adaptive array output signal;first multiplier means coupled to receive the envelope detector meansoutput signal and coupled to receive the adaptive array output signalfor generating the product of the envelope detector means output signaland said adaptive array output signal; and subtractor means that iscoupled to receive the first multiplier means output signal and coupledto receive the adaptive array output signal for subtracting the adaptivearray output signal from the first multiplier means output signal forgenerating a feedback signal; said adaptive array means also including asecond multiplier means for each weighting means coupled to receive thefeedback signal and coupled to receive the corresponding weighting meansinput signal, the output signal of which is coupled to a correspondingintegrator means whose output signal is coupled to the correspondingweighting means for automatically redefining the weight factors.
 24. Asystem as in claim 23 further includes a phase adjustment means foradjusting the phase of the feedback signal.
 25. A signal processingsystem for reducing distortion effects in communication receivers due tointerference signals with signal amplitudes less than the signalamplitude of the signal of interest, said system comprising:adaptivefilter means having:one signal input element; weighting means coupled tothe signal input element for selectively weighting the received signalby a selected weight factor; and summing means for adding together thesignals from the weighting means for generating an adaptive filteroutput signal; biased envelope detector means coupled to receive saidadaptive filter output signal for generating the biased amplitudeenvelope of said adaptive array output signal; and first multipliermeans coupled to receive biased envelope detector means output signaland coupled to receive said adaptive array output signal for generatingthe feedback signal; said adaptive filter means also including a secondmultiplier means for each weighting means coupled to receive thefeedback signal and coupled to receive the corresponding weighting meansinput signal, the output signal of which is coupled to a correspondingintegrator means whose output signal is coupled to the correspondingweighting means for automatically redefining the weight factors.
 26. Asystem as in claim 25 further includes a phase adjustment means foradjusting the phase of the feedback signal.
 27. A signal processingsystem for reducing distortion effects in communication receivers due tointerference signals with signal amplitudes less than the signalamplitude of the signal of interest, said system comprising:adaptivefilter means having:one input signal element; weighting means coupled tothe input element for selectively weighting the received signal by aselected weight factor; and summing means for adding together thesignals from the weighting means for generating an adaptive filteroutput signal; envelope detector means coupled to receive said adaptivefilter output signal for generating the amplitude envelope of saidadaptive filter output signal; biasing means coupled to receive theenvelope detector means output signal for generating a biased envelopedetector means output signal; divider means coupled to receive theadaptive filter output signal and coupled to receive the envelopedetector means output signal which divides the adaptive filter outputsignal by the envelope detector means output signal; and firstmultiplier means coupled to receive biasing means output signal andcoupled to receive divider means output signal for generating a feedbacksignal; said adaptive filter means also including a second multipliermeans for each weighting means coupled to receive the feedback signaland coupled to receive the corresponding weighting means input signal,the output signal of which is coupled to a corresponding integratormeans whose output signal is coupled to the corresponding weightingmeans for automatically redefining the weight factors.
 28. A system asin claim 27 further includes a phase adjustment means for adjusting thephase of the feedback signal.